Two separate Ni(2+) -sensitive voltage-gated Ca(2+) channels modulate transretinal signalling in the isolated murine retina

Retinal Vessel Responses to Flicker Stimulation Are Impaired in Ca v 2.3-Deficient Mice-An in-vivo Evaluation Using Retinal Vessel Analysis (RVA)

Objective: Metabolic demand increases with neuronal activity and adequate energy supply is ensured by neurovascular coupling (NVC). Impairments of NVC have been reported in the context of several diseases and may correlate with disease severity and outcome. Voltage-gated Ca2+-channels (VGCCs) are involved in the regulation of vasomotor tone. In the present study, we compared arterial and venous responses to flicker stimulation in Cav2.3-competent (Cav2.3[+/+]) and -deficient (Cav2.3[-/-]) mice using retinal vessel analysis. Methods: The mice were anesthetized and the pupil of one eye was dilated by application of a mydriaticum. An adapted prototype of retinal vessel analyzer was used to perform dynamic retinal vessel analysis. Arterial and venous responses were quantified in terms of the area under the curve (AUCart/AUCven) during flicker application, mean maximum dilation (mMDart/mMDven) and time to maximum dilation (tMDart/tMDven) during the flicker, dilation at flicker cessation (DFCart/DFCven), mean maximum constriction (mMCart/mMCven), time to maximum constriction (tMCart/tMCven) after the flicker and reactive magnitude (RMart/RMven). Results: A total of 33 retinal scans were conducted in 22 Cav2.3[+/+] and 11 Cav2.3[-/-] mice. Cav2.3[-/-] mice were characterized by attenuated and partially reversed arterial and venous responses, as reflected in significantly lower AUCart (p = 0.031) and AUCven (p = 0.047), a trend toward reduced DFCart (p = 0.100), DFCven (p = 0.100), mMDven (p = 0.075), and RMart (p = 0.090) and a trend toward increased tMDart (p = 0.096). Conclusion: To our knowledge, this is the first study using a novel, non-invasive analysis technique to document impairment of retinal vessel responses in VGCC-deficient mice. We propose that Cav2.3 channels could be involved in NVC and may contribute to the impairment of vasomotor responses under pathophysiological conditions.

Submicromolar copper (II) ions stimulate transretinal signaling in the isolated retina from wild type but not from Cav2.3-deficient mice

BACKGROUND

So far, only indirect evidence exists for the pharmacoresistant R-type voltage-gated Ca²⁺ channel (VGCC) to be involved in transretinal signaling by triggering GABA-release onto ON-bipolar neurons. This release of inhibitory neurotransmitters was deduced from the sensitivity of the b-wave to stimulation by Ni²⁺, Zn²⁺ and Cu²⁺. To further confirm the interpretation of these findings, we compared the effects of Cu²⁺ application and chelation (using kainic acid, KA) on the neural retina from wildtype and Ca_v2.3-deficient mice. Furthermore, the immediately effect of KA on the ERG b-wave modulation was assessed.

METHODS

Transretinal signaling was recorded as an ERG from the superfused murine retina isolated from wildtype and Ca_v2.3-deficient mice.

RESULTS

In mice, the stimulating effect of 100 nM CuCl₂ is absent in the retinae from Ca_v2.3-deficient mice, but prominent in Ca_v2.3-competent mice. Application of up to 3 mM tricine does not affect the murine b-wave in both genotypes, most likely because of chelating amino acids present in the murine nutrient solution. Application of 27 μM KA significantly increased the b-wave amplitude in wild type and Ca_v2.3 (-|-) mice. This effect can most likely be explained by the stimulation of endogenous KA-receptors described in horizontal, OFF-bipolar, amacrine or ganglion cells, which could not be fully blocked in the present study.

CONCLUSION

Cu²⁺-dependent modulation of transretinal signaling only occurs in the murine retina from Ca_v2.3 competent mice, supporting the ideas derived from previous work in the bovine retina that R-type Ca²⁺ channels are involved in shaping transretinal responses during light perception.

Reciprocal modulation of Cav 2.3 voltage-gated calcium channels by copper(II) ions and kainic acid

Kainic acid (KA) is a potent agonist at non-N-methyl-D-aspartate (non-NMDA) ionotropic glutamate receptors and commonly used to induce seizures and excitotoxicity in animal models of human temporal lobe epilepsy. Among other factors, Ca_v 2.3 voltage-gated calcium channels have been implicated in the pathogenesis of KA-induced seizures. At physiologically relevant concentrations, endogenous trace metal ions (Cu²⁺ , Zn²⁺ ) occupy an allosteric binding site on the domain I gating module of these channels and interfere with voltage-dependent gating. Using whole-cell patch-clamp recordings in human embryonic kidney (HEK-293) cells stably transfected with human Ca_v 2.3d and β₃ -subunits, we identified a novel, glutamate receptor-independent mechanism by which KA can potently sensitize these channels. Our findings demonstrate that KA releases these channels from the tonic inhibition exerted by low nanomolar concentrations of Cu²⁺ and produces a hyperpolarizing shift in channel voltage-dependence by about 10 mV, thereby reconciling the effects of Cu²⁺ chelation with tricine. When tricine was used as a surrogate to study the receptor-independent action of KA in electroretinographic recordings from the isolated bovine retina, it selectively suppressed a late b-wave component, which we have previously shown to be enhanced by genetic or pharmacological ablation of Ca_v 2.3 channels. Although the pathophysiological relevance remains to be firmly established, we speculate that reversal of Cu²⁺ -induced allosteric suppression, presumably via formation of stable kainate-Cu²⁺ complexes, could contribute to the receptor-mediated excitatory effects of KA. In addition, we discuss experimental implications for the use of KA in vitro, with particular emphasis on the seemingly high incidence of trace metal contamination in common physiological solutions.

Unconjugated bilirubin modulates neuronal signaling only in wild-type mice, but not after ablation of the R-type/Cav 2.3 voltage-gated calcium channel

INTRODUCTION

The relationship between blood metabolites and hemoglobin degradation products (BMHDPs) formed in the cerebrospinal fluid and the development of vasospasm and delayed cerebral ischemia (DCI) after aneurysmal subarachnoid hemorrhage (aSAH) has been the focus of several previous studies, but their molecular and cellular targets remain to be elucidated.

METHODS

Because BMHDP-induced changes in Ca_v 2.3 channel function are thought to contribute to DCI after aSAH, we studied their modulation by unconjugated bilirubin (UCB) in an organotypical neuronal network from wild-type (WT) and Ca_v 2.3-deficient animals (KO). Murine retinae were isolated from WT and KO and superfused with nutrient solution. Electroretinograms were recorded before, during, and after superfusion with UCB. Transretinal signaling was analyzed as b-wave, implicit time, and area under the curve (AUC).

RESULTS

Superfusion of UCB significantly attenuated the b-wave amplitude in the isolated retina from wild-type mice by 14.9% (P < 0.05), followed by gradual partial recovery (P = 0.09). Correspondingly, AUC decreased significantly with superfusion of UCB (P < 0.05). During washout, the b-wave amplitude returned to baseline (P = 0.2839). The effects of UCB were absent in Ca_v 2.3-deficient mice, lacking the expression of Ca_v 2.3 as proofed on the biochemical level.

CONCLUSIONS

Ex vivo neuronal recording in the murine retina is able to detect transient impairment of transretinal signaling by UCB in WT, but not in KO. This new model may be useful to further clarify the role of calcium channels in neuronal signal alteration in the presence of BHMDPs.

Evaluation of micro Electroretinograms Recorded with Multiple Electrode Array to Assess Focal Retinal Function

Full-field electroretinograms (ERGs) are used to objectively assess the mass function of the retina, whereas focal ERGs are used to evaluate the focal retinal function. The purpose of this study was to determine the usefulness of a multiple electrode array (MEA) system for recording ex vivo micro ERGs (mERGs) together with multiunit spike responses of the retinal ganglion cells (RGCs) to assess focal retinal function in isolated mouse retinas. The a- and b-waves of the full-field ERGs were present in the mERG. The b-wave was blocked by L-AP4, an inhibitor of the mGluR6 receptor, and the OFF-component was blocked by exposure to PDA, an antagonist of ionotropic glutamate receptors, with a corresponding RGC responses. mERGs were also recorded from mice with progressive retinal degeneration, the C57BL/6J-Pde6b^rd1-2J/J (rd1) mice, from which conventional full-field ERGs are non-recordable. A blockade of the glutamate receptors indicated that the negative wave of rd1 mice do not originate from the photoreceptors but from the second or third order neurons. This technique of recording mERGs will be useful in assessing the focal properties of the retinas obtained from eyes with pathology and also to follow the recovery of the physiology of the retina in regenerative studies.

Low concentrations of ethanol but not of dimethyl sulfoxide (DMSO) impair reciprocal retinal signal transduction

BACKGROUND

The model of the isolated and superfused retina provides the opportunity to test drugs and toxins. Some chemicals have to be applied using low concentrations of organic solvents as carriers. Recently, E-/R-type (Cav2.3) and T-type (Cav3.2) voltage-gated Ca(2+) channels were identified as participating in reciprocal inhibitory retinal signaling. Their participation is apparent, when low concentrations of NiCl2 (15 μM) are applied during superfusion leading to an increase of the ERG b-wave amplitude, which is explained by a reduction of amacrine GABA-release onto bipolar neurons. During these investigations, differences were observed for the solvent carrier used.

METHODS

Recording of the transretinal receptor potentials from the isolated bovine retina.

RESULTS

The pretreatment of bovine retina with 0.01 % (v/v) dimethylsulfoxide did not impair the NiCl2-mediated increase of the b-wave amplitude, which was 1.31-fold ± 0.03 of initial value (n = 4). However, pretreatment of the retina with the same concentration of ethanol impaired reciprocal signaling (0.96-fold ± 0.05, n = 4). Further, the implicit time of the b-wave was increased, suggesting that ethanol itself but not DMSO may antagonize GABA-receptors.

CONCLUSION

Ethanol itself but not DMSO may block GABA receptors and cause an amplitude increase by itself, so that reciprocal signaling is impaired.

Cardiac functions of voltage-gated Ca(2+) channels: role of the pharmacoresistant type (E-/R-Type) in cardiac modulation and putative implication in sudden unexpected death in epilepsy (SUDEP)

Voltage-gated Ca(2+) channels (VGCCs) are ubiquitous in excitable cells. These channels play key roles in many physiological events like cardiac regulation/pacemaker activity due to intracellular Ca(2+) transients. In the myocardium, the Cav1 subfamily (L-type: Cav1.2 and Cav1.3) is the main contributor to excitation-contraction coupling and/or pacemaking, whereas the Cav3 subfamily (T-type: Cav3.1 and Cav3.2) is important in rhythmically firing of the cardiac nodal cells. No established cardiac function has been attributed to the Cav2 family (E-/R-type: Cav2.3) despite accumulating evidence of cardiac dysregulation observed upon deletion of the Cav2.3 gene, the only member of this family so far detected in cardiomyocytes. In this review, we summarize the pathophysiological changes observed after ablation of the E-/R-type VGCC and propose a cardiac mechanism of action for this channel. Also, considering the role played by this channel in epilepsy and its reported sensitivity to antiepileptic drugs, a putative involvement of this channel in the cardiac mechanism of sudden unexpected death in epilepsy is also discussed.